A fuel cell supplies hydrogen or a hydrogen-rich gas to one of electrodes between which an electrolyte is interposed, and also supplies an oxidant gas, such as air containing oxygen, to the other of the electrodes, thereby generating electric power through an electrochemical reaction. Recently, attention is focused on a cogeneration system utilizes electric power generated by a fuel cell and also recovers heat that develops when the fuel cell generates electric power, to thus utilize the recovered heat as heat energy.
In the fuel cell system, as one method for generating a hydrogen-rich gas required for a fuel cell, a hydrocarbon raw gas, such as a town gas or an LPG, is subjected to steam reforming along with steam in a reforming unit filled with a reforming catalyst at about 700° C., thereby generating a reformed gas containing hydrogen as a major component. During the steam reforming, carbon monoxide in an amount of about 10% to 15% contained in the reformed gas output from the reforming unit is generated at this time as a by-product. Carbon monoxide poisons an electrode catalyst of the fuel cell, thereby deteriorating power generation capacity. For this reason, it is necessary to eliminate carbon monoxide in the reformed gas up to a concentration of 100 ppm or less and, preferably, 10 ppm or less.
Generally, a shift unit and a selective oxidation unit are provided as a carbon monoxide decreasing unit at a downstream of the reforming unit. The shift unit is filled with a shift catalyst. The shift catalyst causes carbon monoxide in a reformed gas output from the reforming unit to react with steam, thereby performing the water gas shift reaction to form hydrogen and carbon dioxide. The selective oxidation unit is filled with a selective oxidation catalyst. The selective oxidation catalyst is supplied air and a reformed gas of which carbon monoxide concentration has been decreased by the shift unit, thereby subjecting carbon monoxide and oxygen in the air to a selective oxidation reaction such that concentration of carbon monoxide in the reformed gas is decreased to 10 ppm or less. At this time, the shift unit performs the water gas sift reaction at a temperature of about 200° C. or more, and the selective oxidation unit performs the selective oxidation reaction at a temperature of about 100° C.
The reforming unit further includes a heating burner unit. During power generation of the fuel cell system, the burner unit burns hydrogen in the reformed gas that has not been used in power generation of the fuel cell (hereinafter described as an “off-gas”) using the air supplied to the burner unit, thereby maintaining at about 700° C. the temperature of the reforming catalyst for a reforming reaction which is an endothermic reaction. Moreover, during a start-up operation of the fuel cell system, the burner unit burns a raw gas not yet used for generating hydrogen and a mixed gas containing the raw gas and hydrogen, thereby increasing the temperature of the reforming catalyst.
Hereinafter, a hydrogen production device in which a reforming unit equipped with the burner unit, the shift unit, and the selective oxidation unit are connected is described as a fuel processor, when necessary.
At the start-up operation of the fuel cell system, it is necessary to heat the catalysts in the fuel processor to predetermined temperatures for generating a reformed gas from the raw gas. There is disclosed a method which includes: supplying a raw gas to the fuel processor; returning the raw gas output from the fuel processor to the burner unit through a channel bypassing the fuel cell, thereby burning the gas; and heating the catalysts of the fuel processor by combustion heat (see, for example, Patent Document 1).
When power generation of the fuel cell system is halted, supply of the raw gas and steam to the reforming unit is suspended. An interior of the fuel processor is at this time depressurized by volume shrinkage due to a temperature fall of a reformed gas still remaining in the fuel processor and the condensation of steam in the reformed gas due to the temperature fall. In order to avoid such depressurization, when stopping operation, supply of the raw gas and the steam, is first suspended, and after the temperature of the fuel processor has fallen to a predetermined temperature, the reformed gas in the fuel processor is purged by the raw gas. When the internal pressure of the fuel processor has fallen to a predetermined pressure level or less, the raw gas is supplied to the fuel processor, thereby maintaining positive pressure (see, for example, Patent Document 2).